Negative gravity hydraulic system



Sept. 26, 1961 s. R. BARR ETAL NEGATIVE GRAVITY HYDRAULIC SYSTEM 2Sheets-Sheet 1 Filed Aug. 13, 1959 Sept. 26, 1961 s. R. BARR ETAL3,001,368

NEGATIVE GRAVITY HYDRAULIC SYSTEM Filed Aug. 15, 1959 2 sheets-sheet 2United States Patent O ice assgnors to General Electric Company, acorporation of New York Filed Aug. 13, 1959, ser. No. 533,586 claims.(ci. ca -s3) Our invention relates to negative gravity hydraulicsystems, and in particular to a negative gravity system for hydraulicconstant speed drives.

As is well known in the art, a hydraulic constant speed drive is adevice which -accepts a variable speed input and, through a variableratio `transmission formed in part by a hydraulic transmission element,provides a constant speed output. Devices of this kind are commonly usedin aircraft to drive alternating current generators at `a constantspeed. In such an installation the drive is usually mounted on anaccessory gear box of a jet engine and is connected to be drivenmechanically by the engine.

The hydraulic transmission portion of this type of drive requires asupply of hydraulic lluid, usually oil, to make up for leakage flow fromthe hydraulic element and to provide cooling. The main oil supplyreservoir of the engine is normally used as the supply source for thedrive with the engineoil cooling system forming the heat sink for thesystem. In order for the drive to perform its function as a variableratio transmission, it is necessary to supply hydraulic uid to thehydraulic element of thedrive on a substantially continuous basis norder to make up for leakage in the drive. One of the problems that iscommonly encountered in this regard arises out of the interruption inthe engine hydraulic supply to `the drive Which may occur duringnegative gravity conditions induced either by Hight maneuvers or byturbulence.

It is the usual practice to make at least some provision in the engineoil supply system to the drive for negative G operation. Sucharrangements may typically take the form of a baffled supply tankcapable of trapping a portion of the oil in the tank and retaining it incommunication with the supply pump inlet during negative G operation.However, such systems usually can supply hydraulic iluid for only a verylimited time and are intended only to provide continued ilow throughnegative G transient conditions of relatively short time duration. Ithas been considered desirable therefore, and `in some applications arequirement, to provide in the drive itself an emergency system capableof sustained operation for a relatively long period of time undernegative G conditions where the flow of hydraulic iluid from the mainsupply is interrupted.

Accordingly, it is a primary object of our invention `to provide in ahydraulic drive an improved emergency hydraulic system capableofoperation for'relatively long 3,0@1368 Patented Sept. 26, 1961 airbleed system does permit, however, the restoration of the system to itsnormal configuration upon the generation of hydraulic pressure of apreselected magnitude at the pump discharge. *In one arrangement of oursystem, we provide additional hydraulic fluid storage in the drive whichis supplemented by a further quantity of hydraulic Huid which our systemis capable of introducing into the drive during the transient conditionunder which the emergency system is actuated.

Our invention will be further described, and various other advantagesand objects thereof will be pointed out in the following descriptiontaken in connection with the accompanying drawings in which:

FIG. 1 is a schematic presentation of a hydraulic drive having ahydraulic supply and negative gravity system embodying our inventionand,

FIG. 2 is a cross-sectional View of the drive of FIG. l showing theinternal storage and oW passage arrangement in greater detail.

Referring now to FIG. l there is shown in schematic form a hydraulicconstant speed drive having an outer casing 10 provided with an internalreservoir 11. The drive includes a supply pump 12 and a scavenge pump13, both of which are mechanically coupled to and driven by the constantspeed output of the drive. Hydraulic uid, in the usual case oil, issupplied to the drive from a main supply reservoir or tank 14 which ismounted remotely from the drive and which normally forms the main supplysource for the engine on which the drive is mounted. Oil is supplied tothe drive through a boost pump 15 and conduit means 16 connected to theinlet of the supply pump 12 of the drive. The drive casing 10 is ventedto the main oil tank 14 through conduit 17 so that the internal pressurein the casing remains substantially at the atmospheric level.

High pressure oil discharged from the supply pump 12 passes through aconduit 18, a filter 19', which has in series with it a pair of checkvalves 20 and 21, the purpose of which will be described later, and thenthrough a conduit 22 to the pintle 23 of the hydraulic unit, which isthe point in the drive where makeup oil is introduced to compensate forleakage in the system. The manner in which the oil is introduced intothe pintle and the internal oil ow arrangement therein will be set forthlater in connection with the description of FIG. 2, although it will beunderstood that this is merely supplementary material which isintroduced to permit a better understanding of our invention and that itforms no part of it.

The ow rate of oil discharged from the supply pump 12 is substantiallygreater than that required to supply the normal leakage flow to thepintle, although tlow rates considerably greater than the leakage iioware required during system transients created by load and input speedchanges on the drive. Therefore, under steady state operatingconditions, it is necessary to bypass a substantial portion of thesupply pump discharge ow around the periods `of time undernegativegravity conditions with the llow from the main hydraulic supply systeminterrupted.

Briefly stated, we accomplish this, in one form of our invention, by theprovision of a iluid pressure sensitive valving system which detects thereduction in lluid supply pressure accompanying an interruption or decayin flow fromthe main supply and which resets the drive system to anvemergency conguration in which hydraulic iluid is recirculated withinthe drive. We :also provide an air bleed system which is automaticallyactuated to bleed ofi the air discharged from the drive pump duringemergency operation, thereby preventing the air pressure which wouldotherwise be developed by the pump from restoring the system to'fitsnormal configuration. The

pintle. This is accomplished by a bypass conduit 24, which has in serieswith it a regulating valve 25. The regulating valve Z5 includes aplunger type valve member 26 loaded by a spring 27 and set to allowbypass flow at a pressure which in the embodiment shown is in thevicinity of 18() p.s.i. Oil discharged from the bypass conduit 24 isdirected into a storage space 28 which is formed within the drive casingby a wall or bulkhead 29, which in this case also forms the supportmember for the pintle 23. The lower portion of the wall 29 also forms aninlet passage 30 for the scavenge pump 13, so that oil spilling over thetop of the wall 29 and into the chamber 31 formed in the casing on-theright hand side of the wall 29 falls to the bottom ofthe casing 3 and isintroduced to the scavenge pump 13 through inlet 30.

The bypass system is provided with an overpressure relief valve 32,which is connected to the conduit 22 through a conduit 3%` and whichcomprises a plunger 34 loaded by a spring 35 and set to provide bypassilow at an excess pressure level, in the case shown at about 30() p.s.i.By way of example, the pressure relief valve 32 could be expected tooperate under startingy conditions at very low temperatures where thebypass 'valve 25 would be incapable of providing sufficient bypasscapacity at the high oil viscositiesy encountered under theseconditions. Oil discharged from the relief valve 32 is also directedinto the cha'mber 28 on 'the left side of the vwall 29.

The leakage flow from Ythe hydraulic unit, which is relatively small incomparison to the bypass ow, is allowed to fall directly int'o thechamber 31 on the right side ofthe Wall 29. Under 'normal operatingconditions, the bypass :flow therefore keeps the chamber 28 filled withoil, the excess '-ow spilling over the top ofthe wall 29 and returningto the inlet 30 of Vthe scavenge pump. Oil 'is Vscav'enged from thechamber 31, however, Which therefore runs'sub'stantially dry.

The s'cavenge: pump 13 discharges into the reservoir 11 'through aconduit 3'6 which passes through the wall ofthe reservoir. The reservoir11 is provided with a pressurizing valve 38 which comprises aplungerva'lve 39 resiliently loaded by a spring 40 set to permit thedischarge of oil from the reservoir through a discharge opening 441 whentheuid pressure in the reservoir 11 exceeds a preselected level, whichin 'the embodiment presented is in the vicinity of 130 p.s.i.g. Aconduit connects 'the internal case pressure to the opposite side of theplunger 39 so that the valve responds to the `difference lbetweenreservoir pressure and drive case pressure. The reservoir 11 is alsoprovided with an `overpressure relief valve 42 which allows fluid tobe'discharged from kthe reservoir through a conduit 43y into the chamber31 when the pressure exceeds some preselected level in excess of thatwhich the regulating valve 3S is set to maintain. In the embodimentshown, the relief valve 42 is set to permit flow from the reservoir '11to the chamber 31 at pressures in excess of about 200 p.s.i.g. Therelief valve 42 comprises a piston 44 loaded by a spring 4S and`includes a conduit 46 connectingcthe internal drive casing pressure toone side of the piston so that the piston is caused to respond to thedifferences between reservoir pressure and case pressure.

Oil Apassing through the pressurizing valve 39 is directed out of thereservoir through a conduit 47 which has connected in series with it apressure responsive negative gravity valve 48. After passing through thepressure responsive valve 48, the oil is returned to the main supplytank 14 of the system).

'Ihe pressure responsive valve 4S comprises an outer casing 49, havinginlet and discharge ports 50 and 51 respectively connected in serieswith the conduit 47, a spool V52 slidable in the casing and having lands53 and 54 thereon, and a "spring 55 bearing against one end of the spoolas shown. The chamber 6 formed by the casing and one end of 'the'spool52 is vented to atmospheric pressure'through an opening 557 and thechamber 5S formedby the casing and the other end of the spool isconnected 'to the discharge conduit 18 of the Ysupply pump 12through'conduit 59. The longitudinal position of the spool 52 in thecasing is thus a function of the difference between the supply pressureof the pump'and atmospheric pressure. The valve 43 is designed vsuchthat for the normal range of discharge pressures'of the supply pump 12,the spool 52 is in approximately the position illustrated in FIG. 1,with the lands '53 and154 being cleary ofthe inlet and discharge -portsk50' fand 51, thereby allowing unobstructed flow through the valve 48.The spool 52 is provided with an air bleed conduit '60 which extendsthrough the spool from the `chamber 58 and radially out through the land54. The function of the air bleed conduit A60 will be explained later.

The -system is provided with an emergency line 61 having in seriestherewith a check valve 62 which allows ow to pass from the reservoir 11Vto the conduit 22 but blocks ow in the opposite direction. Since undernor mal operating conditions, the pressure in the conduit 22 established`by the regulating valve 25 is higher 'than that established in thereservoir 11 by the pressurizing valve 38, the check valve 62 willnormally remain closed. However, in the event the pressure in the line22 falls below that of the reservoir, the checkvalve Vv62 opens topermit flow from the reservoir throughvthe line 61 into the line 22.'Ihe pressure in the line 22 may fall below reservoir pressure either byreason of a failure in the hydraulic supply system or because, undercertain severe transient conditions creating a very high ow demand, thecapacity of the supply pump 12 is inadequate to supply the full flowrequirement. Thus, Vthe capacity of vthe scavenge pump 1'3, whichmaintains hydraulic pressure in the reservoir 11, is made available fortransient and emergency ow operation.

Upon the occurrence of a negative gravity condition, the hydraulic fluidin the main tank 14 moves to what is normally the top of the tank. Thetank 14 is provided with a bafe 63, however, which traps a certain.quantity of the hydraulic fluid as it moves in the direction of the topof the `tank to maintain a limited supply of the fluid in contact withthe inlet conduit 16a. Thus, vthe Vsystem is provided with a limitedsupply o'f 'hydraulic -iluid which is available under negative gravityconditions.

The arrangement of the various elements within the drive and theinternal oil flow configuration is shown in greater detail in FIG. 2."Referring to FIG. 2, the hydraulic/element is shown at 64 and comprisesan input shaft -65 connected to drive an elliptical race 66 and acylinder block V67 integrally formed with the race -66. The cylinderblock =67 is .provided with la longitudinal extension 68, on which thereis rotatably mounted a cylinder block 69. The cylinder block x69' isconnected to an internally splined output shaft 70 throughinterconnected elements 71, 72, 73, vand 'a unidirectional clutch 74.The supply .pump 12 and the scavengepump '13 as well as various otheraccessories, such as `speed governor, are driven Vby the output.shafting of the .drive .through suitable gearing, not shown. l Y

The cylinder block w67 is provided with a series of radial cylinders .75having ballpistons slidably 4mounted therein. yThe cylinder block .692isof similar construction, having :ajseries of hall pistons 76 slidablymounted in cylinders 77 Vfor radial reciprocating movement therein. Oilis supplied to the .pintle `23 through an linternal -passage 78 from thedischarge line 22 of the supply Vpump 12 and is ducted through internalporting to thecylinder blocks 67 and `69 as shown. Furtherdescription-of the operation of the hydraulic unit -64 is not deemednecessary since the construction and-operation of suchdevices are welllknown lin the art and such additional material `is not essential to anunderstanding of our invention. The operation-of the systemdescribedabove -underfneg tive gravity or emergency conditions will nowbe set forth.

As has been stated above, upon the voccurrence of Ya negative gravitycondition, the bale63 trapsra -limited quantity of fluid in the vicinityxof thevpump inlet 16a. It will be observed vby reference to lFIGS. '1and v2 that by reason of the location of Vthe scavenge pump inletpassage 30 at the lower portion of the-drivefcasing, `the occurrence ofa negative 'gravity condition, in causing the hydraulic fluid infthe'drive to :move to ftheftop ofthe casing .and-away Vfrom the pumpinlet, tprevents the `pump A13 from returningto -the reservoir fl-1Yanyfofthe fluidin the casing.` lUnder these conditions, however,fthelsupply pump 12 continues to pump fluid into the drive.

is exceeded, by the reservoir relief valve 42.

georges Therefore, as long as the fluid delivered to the drive remainsout of contact withthe'scavenge pump inlet 39, the supply pump 12 willcontinue to pump additional hydraulic iluid into the drive casing fromthe emergency Supply contained in the baille 63. Thus, during thisinitial period, the drive takes on or gulps an additional quantity ofhydraulic fluid over and above that normally stored in the drive inanticipation of sustained operation under negative gravity conditions.`As this phase of the cycle proceeds, the oil level in the drive reachesthe point Where it comes into contact with the exposed rotating elementsof the hydraulic unit, i.e. the race 66 and the cylinder block 67. Whenthis occurs, the oil in the casing is set into motion around the insideof the casing by reason of the contact with the rotating parts. 'Iheresulting vortex or swirl motion of the oil in the casing brings it intocontact Wtih the inlet 30 of the scavenge pump 13 and the scavenge pumpagain begins to pump oil from the casing into the reservoir 11, thepressurizing valve 318 again allowing oil to return to the mainreservoir 14 through the pressure responsive valve 48. It will beobserved that during this initial period of negative gravity operation,`the drive automatically takes on a sulicient quantity of additional oilto allow the Setting up of the oil vortex within the drive to therebyreestablish fluid communication with the inlet 30 to the scavenge pumpunder negative gravity conditions. In other words, a sufiicient quantityof additional oil is taken into the drive to supplement that alreadycontained in the chamber 28 to maintain the circulation of oil throughthe scavenge pump during operation under negative gravity conditions.

Upon depletion of the reserve oil supply stored in the bae chamber 63,thepressure at the discharge line 18 of the supply pump decreasesrapidly, causing a corresponding decrease in pressure in chamber 58 ofthe pressure responsive valve 48. This allows the spring 55 to force thespool 52 against the upper limit of the casing 49, bringing the land 54into a position `blocking flow between the inlet and discharge ports 50`and 51, thus shutt ting oir" flow from the reservoir 11 to the maintank 14. It will be observed that when this occurs, the pressurizingvalve 38 is no longer capable of regulating the pressure in thereservoir 11 and that the pressure in the reservoir will therefore beregulated at the higher pressure level established by the relief valve42.

At the same time that the decrease in the discharge pressure of the pump12 causes the valve 48 to close off flow to the main tank 14, thedecrease in pressure in line 22 below that existing in the reservoir 11allows the check valve 62 to open, thereby permitting flow from thereservoir 11 through the emergency line 61 and the check valve 62 to theline 22. The check valves 20 and 21 act to prevent the application ofreservoir pressure back through lines 18 and 59, a condition which wouldotherwise cause the valve 48 to re-open and allow flow from thereservoir 11 back to the tank 14 thereby gradually depleting the oilsupply stored in the drive. Only one of the two check valves 20 and 21is required to perform this function, there being two provided in theparticular embodiment illustrated in order to block flow from the lines18 and 22 when the `filter 19 is removed for maintenance purposes.

With the drive system reset as just described for internalrecirculation, hydraulic fluid is supplied to the pintle line 22 throughthe emergency line 61 from the pressurized reservoir 11. Under theseconditions, the pressure in the reservoir 11 is regulated by theregulating valve 25 and, to the eXteut the capacity of the valve 25 Inorder to maintain'the reservoir pressure at the desired level, it isnecessary that the scavenge pump 13 continue to supply oil to thereservoir through its discharge line 36 at a rate equal to or greaterthan that at which oil -is removed through the emergency line 61; Underthese conditions the pressure in the reservoir 11 will normally beestablished by the reservoir relief valve 42. 'Ihe vortex of VAoil setup within the drive casing during the initial period of negative gravityoperation by the taking on of additional oil into the drive maintainsthe supply of oil to the scavenge pump inlet 30. The oil vortex iscontinuously supplemented by oil discharged back into the casing fromthe regulating valve 42, the bypass valve 25 and by leakage from thehydraulic system so that an internal recirculation system is formedwhich is capable of continuous operation under the conditions justdescribed. In other words, the amount of oil in the casing remainssuti'icient to maintain the oil level in contact with the rotatingelements of the hydraulic unit so that the vortex which feeds thescavenge pump 13'is maintained so long as the negative gravity conditionexists.

Referring to FIG. 1, it will be observed that the movement of the spool`52 of the valve 48 against the upper extremity of the casing aligns theradial portion of the air bleed passage 60 with the discharge port 51.This allows air to be bled from the discharge line 18 of the supply pump12 through the bleed passage 60 back to the main tank 14, `therebypreventing the gradual buildup of air pressure which would otherwise bedeveloped by the Acontinued running of the supply pump and which wouldcause the valve 48 to reopen and allow oil to escape from the reservoir11, and which would also result in pressurization of the drive casing10.

Thesystem will continue to operate in the manner just Vdescribed -aslong as the negative gravity condition remains. When the -system isreturned to normal gravity conditions, the oil supply in the main tank14 is again brought into contact with the pumpinlet 16a and becomesavailable to the supply pump 12. The crosssectional area of the airbleed passage 60 is suiciently small such that while it can accommodatethe necessary flow of air to prevent the pressure in the discharge line1S from building up beyond an undesired level when the supply pump 15 ispumping air, the amount of liquid ow it can accommodate is small incomparison to the capacity of the supply pump. Therefore, when hydraulicfluid is made available to the supply pump, the pressure in thedischarge line 1S builds up to the level established by the regulatingvalve 25, the spool 52 being forced back` to the position shown in FIG.1 in the process, thus once again permitting the reservoir pressurizingvalve 3'8 to return iiuid from the reservoir 11 to the main supply tank14. The return of the reservoir pressure to a level lower than thatestablished in the line 22 by the regulating valve 25 causes the checkvalve 62 to close and the system is returned to its normal operatingconfiguration. As the system continues to operate under normalconditions, the excess fluid is scavenged from the chamber 31 and thechamber 28 is again filled with hydraulic fluid in the manner alreadydescribed.

While we have shown and described a particular embodiment of ourinvention for the purpose of making a full disclosure thereof, it is tobe appreciated that our invention is not-limited to the embodiment setforth and that various changes, substitutions and modifications may bemade in the embodiment presented without departing from the true scopeand spirit of our invention as de- `lined in the appended claims.

. What We claim and desire to secure by Letters Patent .of the UnitedStates is:

.sure'responsive valve connected to respond to the duid vpressure insaid fluid system on the discharge side of said 'supply pump, saidpressure responsive valve being connected in series with saidpressurizing valve to shut oi iiow from said reservoir -to said mainsupply source when the pressure on the discharge side of said supplypump falls below a preselected level.

2. ln a system for a hydraulic drive rhaving a .main hydraulic iiuidsource, asupply pump for supplying -fluid under pressure -from said mainsource to the drive, a reservoir associated with the drive, andascavenge .pump connected to scavenge uid from the drive andfdischarge itinto said reservoir; an emergency hydraulic system `comprising areservoir pressurizing 'valve connected to Ireturn iluid to said mainsupply source from said reservoir when the fluid `pressure in saidreservoir exceeds a preselected level, a pressure relief valve connectedto discharge fluid -from said reservoir linto the drive when -thepressure in said reservoir exceeds -a second preselected level higherthan that normally maintained by said pressurizing valve, a .pressureresponsive valve connected 'to respond -to the fluid pressure in .saidlhydraulic drive system at apoint on the discharge side of said supplypump, said pressure responsive valve being connected to shut oi the ilowof uid 'from said reservoir to -the main supply source fin response to adecrease in the discharge pressure of said supply pump below apreselected level, conduit means connecting said reservoir to thehydraulic supply system of the driveat a point downstream of said supplypump, `and valve means permitting lio'w 'from 'said 'reservoir into thedrive supply system through said conduit means when the fluid pressuregenerated in the supply system by said supply pump falls 'below apreselected level.

3. Apparatus as set forth in vclaim 2 including check valve meansconnected between said conduit 'means and said 'pressure 'responsivevalve to vprevent the application of reservoir pressure to said pressureresponsive valve through said conduit means.

4., A fluid system for a hydraulic vdrive 4comprising la supply pump forsupplying tluid under pressure from a main supply source to said drive,a reservoir associated with said drive, a scavenge pump connected toscavenge fluid from said drive land discharge it into the saidreservoir, a .pressurizing valve connected to return 4lluid 'to saidmain supply source when the pressure lin said reservoir'exceeds apreselected level, a pressure responsive valve connected to respond tothe 'fluid pressure in said 'fluid system onthe discharge side kof saidsupply'pump, said pressure reponsive valve being connected to shut otll'ow from said reservoir to said main supply source when the pressureon the vdischarge side of said supply pump falls below a preselectedlevel, and air bleed means kactuated upon movement of said pressureresponsive valve to a position shutting oi ilow from said reservoir tosaid main supply source to bleed olf the air discharged by said supplypump Aunder such conditions.

5. 4'In `a system for a hydraulic ydrive having a Vmain hydraulic duidsource, a supplypump for supplying Huid vrunder pressure from said mainsource to ithe drive, a

reservoir associated with the drive, and a scavenge pump connected toscavenge fluid from the drive and discharge it Yinto said reservoir; anemergency -hydraulic system comprising a reservoir pressurizing valveconnected to'return fluid to said main supply lsource 'from saidreservoir 'when the fluid ypressure in said reservoir exceedsapreselected level, a pressure relief Valve connected to ldisvchargeiluid from said `reservoir'into'the drive -when Ethe pressure insaid'reservoir exceeds Ea 'second preselected level 'high'erth'an thatynormallymaintained by said-pressuri'zing valve, va pressure responsivevalve connected to respond to the `fluid pressurein-saidhydraulic-*drive'systern vatapoint on the discharge side of said supplypump, said 'pressure `responsive valve being 'connected `to -'shutoilthe flow-of Aliluid froms'aid'r'eservoir tothe' mainsupp'ly source inresponseto 'a decrease'inthe discharge presdrive supply system throughsaid conduit means when 'the iiuid pressure generated in the Ihydraulicdrive sys'temby said supply pump falls below 'a preselected level, andair bleed meanskactuated upon movement of 'said 'pressure responsivevalve blocking iiow 'from-said 'reservoir to 'said main supplysource'to'bleed olf 'the air discharged .by said supply pump under suchconditions.

6. I'Apparatus 'as 'set forth rin claim 5 'including check valve meansconnected between said conduit means 'and said pressure 'responsivevalve 'to 'prevent the application o'f reservoir pressure `to saidpressure 'responsive valve through said .conduit means. l

7. `A system 'for 'a hydraulic drive comprising avarible speed ratiotransmission including a rotatable cylinder 'block 'and race assembly,ran outer casing extending around said transmission, a supply pump forsupplying duid under pressure to said transmission from a ylluid source,a scavenge pump connected to scavenge fluid 1from said casing, apartition in `.said casing vextending partially across transmission,means for 'discharging 'the bypassed Vflow into said Llirs't `chamber to'maintainsaid lirst chamber 'substantially full of uid under normaloperatingconditions,

.said transmission being located in said second chamber,

and inlet 'means 'for 'said scavenge pump located in said secondchamber, whereby said transmission normally operates in a lsubstantiallydry chamber, 'the iiuid stored 4in "said iirst chamberv 'being thrown tovthe top fof "said casing under negative gravity'conditions intocommunica- 'tion 'with said passage to become available for iiloodingsaid second chamber, the duid entering said second chamber d coming into`contact with said rotatable cylinder -block vand race assembly andbeing thereby induced into a swirl motion whereby said fluid is :broughtinto contact with the lscavenge vrpump inlet means during negativegravity conditions.

8. A hydraulic drive "fluid system as set forth inclaim 7 includingmeans associated with said fluid source :for :making available to saiddrive under negative gravity conditions a limited quantity of hydraulicfluid to supplefmentthe `fluid stored in said drive.

9. A hydraulic'drive 'fluid system as set forth in claim 7 including areservoir associated with said drive, said fscavenge pump beingconnected `to discharge in'to said reservoir, a pressurizing valveconnected to return uid 'from the 'reservoir t'o the main supply sourcewhen the pressure ins'aid reservoir exceeds a preselected level, a`pressure relief kvalve 'connected to 'discharge -luid from "saidreservoir '-into `the casing when vthe pressure in 4said reservoirexceeds a second Ypreselected level higher than 'that normallymaintained by said pressurizing valve, a

.pressure responsive va'lve connected to respond 'to 'the 1'duit meanswhen the lfluid pressure generated in said drive uidfsystem by thesupply VV.pump falls below the pressu're in said reservoir.

110.:A :uid system Vfor a hydraulic drive comprising a A'supply `pump"foi-:supplying duid 'under pressure ifi-om Sa main supply source tosaid drive, a scavenge system including a scavenge pump for scavenginguid from said drive and returning it to said main supply source, apressure responsive valve connected in series ow re1ati0nship With saidscavenge system and operable to one position blocking return ilow `fromsaid drive to said supply and to a second position permitting saidreturn flow, said pressure responsive valve being connected to respondto the fluid pressure in said fluid system on the discharge side of saidsupply pump, whereby return ow from said drive to said supply sourcethrough said scavenge system is shut off by said pressure responsivevalve when the pressure on the discharge side of said supply pump fallsbelow a preselected level.

No references cited.

